In recent years, with the increase in the demand for information communication, increasing the optical transmission capacity is being requested; however, the increasing demand for information communication cannot be met sufficiently only by increasing the wavelength multiplexing number and newly laying fiber-optic networks. Thus, it is being requested to increase the data transmission rate per carrier signal from 10 Gbps (Gigabits per second) to 40 Gbps or 100 Gbps. However, especially in optical transmission systems performing long-distance transmission, transmission penalties such as distortion of the optical signal waveform due to chromatic dispersion (CD) and polarization mode dispersion (PMD) of the optical fiber as the communication channel become marked with the increase in the data transmission rate.
Communication methods capable of compensating for the influence of such transmission penalties (chromatic dispersion and polarization mode dispersion) by signal processing performed by a digital signal processor of an optical signal reception device and also dealing with various types of multilevel modulation methods have come into practical use. For example, there have been proposed methods employing multilevel modulation such as Dual Polarization-Quadrature Phase Shift Keying (DP-QPSK) and Dual Polarization-Binary Phase Shift Keying (DP-BPSK) as digital coherent communication methods (see Patent Reference 1, for example).
There has also been proposed a multi-carrier system in which one transponder includes a plurality of optical transmission/reception units and data are transmitted by using a plurality of carrier signals having wavelengths different from each other. In cases where the number of the carrier signals is N (N is an integer larger than or equal to 2), one transponder includes N transmission/reception units and data are transmitted by using N carrier signals having wavelengths different from each other.